1. Field of the Invention
The present invention relates to a GaAs semiconductor substrate having a clean surface, conveniently employed as the substrate for various semiconductor devices such as a light emitting device, electronic device, and semiconductor sensor, and a fabrication method of such a GaAs semiconductor substrate.
2. Description of the Background Art
A GaAs semiconductor substrate conveniently employed as the substrate of various semiconductor devices such as a light emitting device, electronic device, and semiconductor sensor has a dangling bond at the surface. Therefore, the surface will be modified due to the adhesion of impurities and formation of oxides at the surface. If a semiconductor layer is grown on a GaAs semiconductor substrate with impurities or oxides at the surface to further produce a semiconductor device, the impurities and/or oxides will be introduced into the semiconductor device to degrade the property thereof.
In view of this issue, a GaAs semiconductor substrate, when a semiconductor layer is to be grown thereon, is heated to approximately 500 to 600° C. in order to remove the impurities and/or oxides present at the surface of the GaAs semiconductor substrate prior to formation of the semiconductor layer (thermal cleaning of the surface of the GaAs semiconductor substrate). However, the surface of the GaAs semiconductor substrate is readily oxidized, so that an oxide that cannot be removed even by the aforementioned thermal cleaning of the surface may be produced. In particular, the melting point of Ga2O3, that is such an oxide of Ga is 1795° C., which is extremely high, and cannot be removed by the general thermal cleaning carried out at approximately 500 to 600° C.
There have been various approaches to provide a GaAs semiconductor substrate having a clean surface, absent of adhesion of impurities and formation of oxides. For example, Japanese Patent Laying-Open No. 07-201689 discloses a semiconductor wafer with a protection film, having a Langmuir-Blodgett film formed on the surface of the GaAs wafer, followed by a coating of a high polymer film thereon. However, the semiconductor wafer with a protection film disclosed in Japanese Patent Laying-Open No. 07-201689 employs a surface active agent formed of a hydrocarbon compound of macromolecules. Therefore, carbon atoms and/or oxygen atoms originating from the surface active agent will remain on the surface of the semiconductor wafer even if thermal cleaning is carried out prior to growth of a semiconductor layer. There was a problem that the property of the semiconductor device is degraded.
Japanese Patent Laying-Open No. 06-124863 discloses a GaAs compound semiconductor substrate subjected to final polishing, wherein the difference between the atomic ratio of gallium to arsenic in the area within 10 nm from the substrate surface (Ga/As) and the atomic ratio of gallium to arsenic at the (110) cleavage plane (Ga/As)C does not exceed ±0.2. However, even if (Ga/As) is set to approximate (Ga/As)C, i.e. the stoichiometric composition ratio, the presence of many Ga at the surface of the substrate will lead to the formation of Ga2O3, that has an extremely high melting point (1795° C.) due to oxidization of Ga at the surface. Therefore, Ga2O3, cannot be removed by the general thermal cleaning that is carried out at approximately 500° C.-600° C.